Device and method for reducing slip in the control system of a cvt in a motor vehicle

ABSTRACT

The invention proceeds from a control of a transmission which is adjustable continuously with respect to its transmission ratio, for a motor vehicle. The transmission together with a drive unit is mounted in the drive train of the motor vehicle and the drive unit has an adjustable drive torque. The transmission includes a drive end as well as an output end and operative means for establishing a mechanical operative connection between the drive end and the output end. Furthermore, detection means for detecting a slip quantity is provided. The slip quantity represents the slip between the operating means and the drive end and/or output end. Pregivable measures are initiated in response to a pregiven value of the slip quantity. The essence of the invention is that measures as follows are initiated: a drive of a clutch mounted in the drive train; and/or, a change of the transmission ratio; and/or, a change of the output torque of the drive unit. It is especially intended that measures are initiated in the sense of a slip reduction.

STATE OF THE ART

[0001] The invention relates to an arrangement and a method forcontrolling a transmission in a motor vehicle having the features of thepreamble of claims 1 and 9. The transmission is continuously adjustablewith respect to its transmission ratio.

[0002] In motor vehicles having a known continuously variabletransmission (CVT) with a continuous element (for example, a thrustelement belt or chain), the contact engaging force of the conicalpulleys against the thrust element belt must be so adjusted via suitablemeasures that the torque, which is to be transmitted, can betransmitted. A high slippage between the continuous element and thepulleys occurs when the contact engaging force is too low and this canlead to damage. If the contact engaging force is, however, greater thanabsolutely necessary to avoid slippage, then the efficiency of thetransmission drops which leads to an unnecessarily high consumption offuel. For this reason, it is advantageous to so select the contactengaging force that the maximum tranmissible torque for this contactengaging force is only slightly greater than the torque to betransmitted at the particular time. If a high slippage of the continuouselement nonetheless occurs, then measures must be taken in order tostabilize or stop the relative movement of the continuous element sothat damage is avoided.

[0003] Conventional continuously variable transmission controls, such asdisclosed in EP,A1, 0 451 887, determine the transmission input torquefrom the torque, which is outputted by the engine, and the converteramplification. A belt tension is computed from this torque from whichbelt tension a contact engaging force results which makes possible areliable transmission of this torque. Here, as a rule, a considerablereserve of reliability is considered which causes a higher fuelconsumption. If an impermissibly high slippage nonetheless occurs, then,in general, the contact engaging force and therefore the belt tension isincreased. The speed with which a reaction can be had to excessiveslippage is, however, limited by the time constants of the hydraulic andof the mechanical system of the belt tensioning adjustment.

[0004] A system is known from EP,B1,0 446 497 (corresponding to U.S.Pat. No. 5,098,345) for protecting against exceeding a maximum slippageof the belt. Here, a clutch is so controlled that this clutch alwaysstarts slipping at lower drive torques than the band. A reaction to apossible slippage of the band is not described here.

[0005] The detection of belt slippage is known in many configurations.Thus, and for example in accordance with DE-OS 44 11 628, it issuggested to detect the speed of the continuous element. Furthermore,the belt slippage can be detected by a sensor which simultaneouslydetects the axial displacement of conical-pulley pair and the rpm ofthis pair. From earlier German patent application 196 38 277.7, it isknown to provide at least two sensor units for slippage detection whichare mounted in the region of the continuous means and between the driveend and the output end.

[0006] The object of the present invention is to provide effectivemeasures for excessive slippage in order to reliably avoid damage to thetransmission.

[0007] This object is achieved with the features of claims 1 and 9.

ADVANTAGES OF THE INVENTION

[0008] As mentioned, the invention proceeds from a control of atransmission, which is continuously adjustable with respect to itstransmission ratio, for a motor vehicle. The transmission is mounted inthe drive train of the motor vehicle together with a drive unit havingan adjustable drive torque. The transmission has a drive end and anoutput end and operative means for establishing a mechanical operativeconnection between the drive end and the output end. Furthermore,detecting means for detecting a slippage quantity is provided whichrepresents the slippage between the operative means and the drive endand/or output end. Pregivable measures are initiated in response to apregivable value of the slip quantity. The essence of the invention isseen in that the following are provided as measures:

[0009] a control of a clutch mounted in the drive train; and/or,

[0010] a change of the transmission ratio; and/or,

[0011] a change of the output torque of the drive unit.

[0012] It is especially intended here that the measures are initiated inthe sense of avoiding slip.

[0013] In an advantageous embodiment of the invention, a further measureis a change of the contact engaging force between the drive end and/orthe output end and the operative means, especially in the sense ofavoiding slip.

[0014] The present invention describes how through slipping of thecontinuous element can be stopped and stabilized to a tolerable slipwith three or four possible interventions when through slip of thecontinuous element is detected. The invention offers the possibility toundertake the introduced measures individually or with a coordinatedstrategy. Depending upon the particular situation, which led to athrough slip of the continuous element, a response is provided inaccordance with a previously determined strategy having a selection ofthe measures according to the invention simultaneously or in a tighttime-dependent sequence. This always takes place with the objective tosuppress a detected slip as quickly as possible and not to confuse thedriver with effects of the measures. Depending upon the equippedvariation of the vehicle, it is possible that not all of these measurescan be carried out.

[0015] It is especially provided that:

[0016] the drive of the clutch takes place in the sense of an opening ofthe clutch; and/or,

[0017] the change of the transmission ratio takes place in response to apositive slip quantity in the sense of an increase of the transmissionratio and in response to a negative slip quantity in the sense of areduction of the transmission ratio; and/or,

[0018] the change of the output torque of the drive unit takes place inresponse to a positive slip quantity in the sense of a reduction of theoutput torque and in response to a negative slip quantity in the senseof an increase of the output torque.

[0019] Furthermore, it is especially advantageous that the following isdependent upon the extent of the detected slip quantity:

[0020] the extent of the drive of the clutch; and/or,

[0021] the extent of the change of the transmission ratio; and/or,

[0022] the extent of the change of the output torque of the drive unit;and/or,

[0023] the extent of the change of the contact engaging force.

[0024] In a further advantageous embodiment of the invention it isprovided that, simultaneously or in time after the drive of the clutchin the sense of an opening of the clutch, the following is provided: achange of the transmission ratio; and/or, a change of the instantaneousoutput torque of the drive unit; and/or, a change of the contactengaging force in such a manner that the maximum transmissible torque bythe transmission becomes again greater in magnitude than the magnitudeof the instantaneous output torque of the drive unit. In this way, theclutch slip can again be reduced by an increase of the maximumtransmissible torque of the clutch (closing of the clutch).

[0025] It is especially provided that the change of the output torque ofthe drive unit for slip reduction is actuated as support to at least oneof the other measures (clutch drive and/or change of the transmissionratio and/or contact engaging force).

[0026] As already described, the transmission can be a continuoustransmission. The drive end and/or the output end can have at least anaxially displaceable element which has essentially the form of a conicalpulley. As operative means, at least a belt (preferably a thrust elementbelt) or a belt or a chain is tensioned between pulley pairs whichdefine the drive end and the output end.

[0027] Further advantageous configurations of the invention can be takenfrom the dependent claims and the embodiments described hereinafter.

DRAWING

[0028]FIG. 1 shows schematically a continuously variable transmissionhaving a known control of the contact engaging force; whereas, FIG. 2shows an embodiment with reference to a block circuit diagram.

EMBODIMENTS

[0029] The invention will now be described in detail in the followingwith respect to the embodiments.

[0030] In FIG. 1, a known configuration of a continuously variabletransmission is shown in section. The internal combustion engine 1 canbe influenced by the throttle flap 2 with respect to its outputtedtorque M_(m). The throttle flap 2 is, for example, coupled mechanicallyor electrically to the accelerator pedal (not shown). The engine 1 iscoupled mostly by means of a clutch and/or a converter 3 to the driveend (primary end) of the CVT transmission 4. The output end (secondaryend) of the CVT transmission 4 is connected via a downstreamtransmission (not shown) to the wheels of the vehicle. The CVTtransmission has an axially displaceable conical pulley on each of theprimary and secondary ends. To adjust the transmission ratio, acorresponding primary pressure P_(p) or a secondary pressure P_(s) isbuilt up in the oil chambers 7 and 8, respectively.

[0031] With a suitable selection of the actuating quantities of primarypressure P_(p) and secondary pressure P_(s), the following must beensured:

[0032] 1. the transmission ratio i corresponds to the desired ratio ofprimary rpm N_(p) and secondary rpm N_(s); and,

[0033] 2. the force transmitting thrust element belt 9 (for example,chain, belt) is pressed against the pulleys with sufficient force inorder to prevent a through slippage of the thrust element 9.

[0034] The above-mentioned point 1 is utilized via an electro-hydraulictransmission ratio or primary rpm control 10. For point 2, a belttension control 11 is used.

[0035] Rpm sensors 12, 13 and 14 are provided on the engine 1 and on theCVT transmission 4 for the transmission ratio and belt tension control.The rpm sensors 12, 13 and 14 detect the engine rpm M_(m), the primaryrpm N_(p), and the secondary rpm N_(s).

[0036] The master-slave principle is shown in FIG. 1 and is mostlyutilized. In this master-slave principle, the secondary pressure P_(s)serves to adjust the belt tension and the primary pressure P_(p) servesto adjust the transmission ratio rpm or the primary rpm. For thealternative partner principle, the belt tension control influences theprimary pressure as well as the secondary pressure.

[0037] Generally, one can state that a position signal in the form of apressure quantity P_(B) is available for the belt tension control. Fromthe literature, several methods for controlling the belt tension areknown, which however all work in a similar manner.

[0038] In FIG. 1, a sensor is identified by reference numeral 18 whichdetects the speed V_(b) of the belt 9. In block 24, the actual belt slipS is determined from the belt speed V_(b), the primary rpm N_(p) and thesecondary rpm N_(s) as described in the initially-mentioned DE-OS 44 11628. It is here noted that the slip determination can also be obtaineddifferently without departing from the concept of the invention.

[0039] In FIG. 2, the four possible interventions for slip reduction areshown. With reference numerals 27, 28 and 29, the drive motor 27 withthe corresponding engine control 22, the clutch 28 with thecorresponding clutch control 23 as well as the CVT transmission 29 withthe already-described slip detection 24, the belt tensioning control 25and the transmission ratio control 26 are shown.

[0040] The individual actions for reducing the through slip of thecontinuous element are matched by the block 21 “slip intervention” inresponse to a detected excessive slip S of the continuous element inorder to stop the slip or to stabilize the slip to an acceptable slip.In dependence upon the particular situation which led to a through slipof the continuous element 9, a response is provided in accordance with apreviously determined strategy with a selection of measures (heredescribed) simultaneously or in a tight time sequence. These measuresare undertaken with the object of suppressing a detected slip as quicklyas possible and not to confuse the driver by the effects of themeasures.

[0041] Not all of these measures can be carried depending upon theequipment variation of the vehicle.

[0042] Increase of the Belt Tension (Block 25):

[0043] If the slip detection 21 detects an increased slip S, then thetension of the continuous element 9 is increased in order to reduce theslip S to a tolerable amount via an increased contact engaging force.The increase ΔF takes place independently of the sign of the detectedslip S.

[0044] It is especially advantageous to select the quantity ΔF of theadditional tension of the continuous element 9 in dependence upon theamount of the detected slip S. Here, it must considered that the contactengaging force must be greatly increased because of the transition fromstatic friction to sliding friction between the continuous element andthe pulley in order to reduce slip. It is therefore advantageous whenthe additional belt tension ΔF is not linearly dependent upon thedetected slip S. As already mentioned, the speed with which a responsecan be made is limited by the time constants of the hydraulic system andof the mechanical system.

[0045] Transmission Ratio Adjustment (Block 26):

[0046] If the slip detection 24 detects an increased slip S, then thetransmission ratio i of the continuous transmission is so adjusted thatthe geometric transmission ratio corresponds again to the rpmtransmission ratio.

[0047] For a known positive slip, this means that the slip interventionrequires a positive additional transmission ratio Δi, that is, a largertransmission ratio is required than without intervention. On the otherhand, for a detected negative slip S, a negative additional transmissionratio ΔI is required which reduces the transmission ratio i of thetransmission.

[0048] It is especially advantageous to select the magnitude of theadditional transmission ratio Δi in dependence upon the detected slip S.

[0049] This intervention for positive slip is especially suitable forthe present-day conventional constructions of the CVTs. For positiveslip, for which the engine torque M_(m) can no longer be transmitted,the engine rpm increases greatly. The additional transmission ratio Δican be realized by releasing oil from the primary pulley without anadditional pumping capacity for hydraulically increasing the contactengaging force. This measure should, however, be combined with areducing engine torque intervention (block 22) which is yet to bedescribed.

[0050] Torque Intervention at the Engine (Block 22, 27):

[0051] If the slip detection 21 detects an increased slip S, then thetorque M_(m), which is outputted by the engine 27, can be changed inorder to adapt the torque, which is to be transmitted by thetransmission, to the maximum transmissible torque M_(max) and to therebyreduce the slip to a tolerable amount.

[0052] For positive slip (that is, when the transmission input rpm N_(p)is greater than it should be in accordance with the geometrictransmission ratio), the torque M_(m), which is outputted by the engine27, must be reduced (the additional torque ΔM_(m) is negative). For thispurpose, all interventions can be used which are applicable to a knownoutput slip control (ASR). These interventions include, for example, anignition angle intervention for a spark-ignition engine not havingelectronic engine power control (E-gas). For engines having electronicengine power control (E-gas or EDC), simply a reduced torque isrequired. In each case, the slip intervention ΔM_(m) requires a negativeadditional torque.

[0053] If the drag torque of the engine 27 with respect to magnitude isgreater than the torque M_(max), which is maximally transmissible by thetransmission, then the transmission input rpm N_(p) is less than itshould be in accordance with the geometric transmission ratio. In thiscase, a negative belt slippage is present and the drag torque of theengine 27 should be reduced. All interventions can serve for thispurpose which are used at the present time for a known engine dragtorque control (MSR). For engines having electronic engine power control(E-gas or EDC), simply a higher torque is required. This means that theslip intervention requires a positive additional torque ΔM_(m) for anegative slip.

[0054] It is especially advantageous in both cases to select themagnitude of the additional torque ΔM_(m) at the engine 27 in dependenceupon the amount S of the detected slip. These interventions arecharacterized by their relatively short time constants and shouldtherefore always be applied supportive to the other interventionspresented in this application.

[0055] Clutch Intervention (Block 23, 28):

[0056] If the vehicle has, for example, an electronically influenceableclutch 28, then, when slip S of the continuous element 9 in thetransmission is detected, the maximum transmissible torque M_(Kmax) ofthe clutch 28 can also be reduced so that the slip occurs at the clutch28 in lieu of at the continuous element 9. This is advantageous becausethe clutch 28 is so configured in its construction that it can withstanda higher slip for a certain time without damage.

[0057] If the maximum transmissible torque M_(K,max) is reduced at theclutch 28 by a clutch intervention ΔM_(K), then the torque at thetransmission input cannot be greater in magnitude than the maximumtransmissible torque M_(K,max) of the clutch. In this way, it ispossible to make the torque of the transmission 29, which is to betransmitted, less in magnitude than the maximum transmissible torqueM_(get,max) of the transmission 29 and to so limit the slip at thetransmission to a noncritical value.

[0058] As long as the torque M_(m) at the engine output, the contactengaging of the continuous element 9 and the transmission ratio i of thetransmission do not change, the slip then occurs at the clutch 28instead of at the transmission 29. This is advantageous for theabove-mentioned reasons.

[0059] It is especially advantageous to ensure, via one of the otherthree described measures, that the torque M_(get,max), which is themaximum torque transmissible by the transmission 29, becomes greateragain in magnitude than the magnitude of the engine output torque inorder to be able to reduce the slip at the clutch 28 via an increase(closure of the clutch) of the maximum transmissible torque M_(K,max) ofthe clutch.

[0060] In summary, it can be stated that the described possibilities canbe applied individually or in combination. Especially, in dependenceupon the situation, combined interventions should always be carried outso that the driver does not notice the interventions on the slipcontrol. Within a few milliseconds (50 to 590 ms), the condition wantedby the driver should again be applicable in the drive train of thevehicle so that the vehicle does not unexpectedly accelerate ordecelerate.

1. Arrangement for controlling a transmission 4, which is continuouslyvariable in its transmission ratio, for a motor vehicle, thetransmission together with a drive unit (1; 27) being mounted in thedrive train of the motor vehicle, the drive unit (1; 27) having anadjustable drive torque M_(m), the transmission having a drive end aswell as an output end and operative means 9 for producing a mechanicaloperative connection between the drive end and the output end; and,detection means 24 for detecting a slip quantity S, which represents theslip between the operative means and the drive end and/or output end;and, means 21 by means of which pregivable measures are introduced inresponse to a pregivable value of the slip quantity; characterized inthat: as measures (23, 26, 22): a drive (ΔM_(K)) of a clutch (28)mounted in the drive train; and/or, a change (Δi) of the transmissionratio (i); and/or, a change (ΔM_(m)) of the output torque of the driveunit (27) is pregiven, wherein it is especially provided that themeasures are initiated in the sense of a slip reduction.
 2. Arrangementof claim 1 , characterized in that, as a further measure (25), a change(ΔF) of the contact engaging force between the drive and/or output endand the operative means, especially in the sense of a slip reduction, isprovided.
 3. Arrangement of claim 1 , characterized in that, as ameasure: the drive (ΔM_(K)) of the clutch (28) takes place in the senseof an opening of the clutch; and/or, the change (Δi) of the transmissionratio (i) takes place in response to a positive slip quantity in thesense of an increase in the transmission ratio and in response to anegative slip quantity in the sense of a reduction of the transmissionratio; and/or, the change (ΔM_(m)) of the output torque of the driveunit (27) takes place in response to a positive slip quantity in thesense of a reduction of the output torque in response to a negative slipquantity in the sense of an increase of the output torque. 4.Arrangement of claim 1 , characterized in that: the extent of the driveof the clutch; and/or, the extent of the change of the transmissionratio; and/or, the extent of the change of the output torque of thedrive unit; and/or, the extent of the change of the contact engagingforce is dependent upon the extent of the detected slip quantity S. 5.Arrangement of claims 1 or 2, characterized in that the transmission hasa maximum transmissible torque (M_(get,max)) and, simultaneously or intime after the drive (Δ_(K,max)) of the clutch in the sense of anopening of the clutch, a change (Δi) of the transmission ratio and/or achange (ΔM_(m)) of the instantaneous output torque of the drive unitand/or, a change of the contact engaging force (ΔF) is made in suchmanner that the torque (M_(get,max)), which is transmitted maximally bythe transmission, becomes greater than the magnitude (M_(m)) of theinstantaneous output torque of the drive unit.
 6. Arrangement of claim 1, characterized in that the clutch (28) responds in the sense of aclosure opening in response to the torque (M_(get,max)), which is themaximum torque transmissible by the transmission, becomes greater inmagnitude than the torque (M_(m)) of the instantaneous output torque ofthe drive unit.
 7. Arrangement of claims 1 or 2, characterized in thatthe change (ΔM_(m)) of the output torque of the drive unit is actuatedfor slip reduction in support to at least one of the other measures(clutch drive and/or change of the transmission ratio and/or of thecontact engaging force).
 8. Arrangement of claim 1 , characterized inthat the transmission (29) is a continuous transmission and the driveend and/or the output end has at least an axially displaceable element(5, 6) which has essentially the form of a conical pulley and that asoperative means (9) at least a belt, preferably a thrust element belt ora belt or a chain is clamped between disc pairs which define the driveand the output ends.
 9. Method of controlling a transmission (4) whichis adjustable continuously with respect to its transmission ratio, for amotor vehicle; the transmission together with a drive unit (1, 27),which has an adjustable drive torque (M_(m)), is mounted in the drivetrain of the motor vehicle; the transmission having a drive end as wellas an output end and operative means (9) for producing a mechanicaloperative connection between the drive end and the output end; and aslip quantity (S) is detected which represents the slip between theoperative means and the drive end and/or output end; and, pregivablemeasures are initiated in response to a pregivable value of the slipquantity; characterized in that: as a measure, especially in the senseof slip reduction, are provided: a drive (ΔM_(K)) of a clutch (28)mounted in the drive train; and/or, a change (Δi) of the transmissionratio (i); and/or, a change (ΔM_(m)) of the output torque of the driveunit (27).
 10. Method of claim 1 , characterized in that, as measures:the drive (ΔM_(m)) of the clutch (28) takes place in the sense of anopening of the clutch; and/or, the change (Δi) of the transmission ratio(i) in response to a positive slip quantity takes place in the sense ofan increase of the transmission ratio and, in response to a negativeslip quantity, takes place in the sense of a reduction of thetransmission ratio; and/or, the change of the output torque (ΔM_(m)) ofthe drive unit (27) takes place in response to a positive slip quantityin the sense of a reduction of the output torque, in response to anegative slip quantity in the sense of an increase of the output torque;wherein it is especially provided that: the extent of the drive of theclutch; and/or, the extent of the change of the transmission ratio;and/or, the extent of the change of the output torque of the drive unitis dependent upon the extent of the detected slip quantity and, as afurther measure, a change of the contact engaging force is providedbetween the drive and/or output end and the operative means, especiallyin the sense of a slip reduction.